Only thing I dislike about the game is that for some reason your fire rate and that of enemies increases if you play eith your phone's battery saver option off and decreases when it's on. I was genuinely tweaking when I beat some levels with fast enemies and some with slower ones.
Other than this weird bug/feature the game is really good. I bet every level with max stars and put myself through the grinder to not take a si gle point of damage.